Avalanche photodiodes (APDs) have applications as very sensitive detectors of optical signals. When operated with a reverse voltage greater than its breakdown voltage, a mode of operation often referred to as “Geiger mode”, an avalanche photodiode can in some instances be used to receive and measure a single photon. A device suitable for such purpose is sometimes referred to as a “single photon avalanche detector” (SPAD).
In the operation of an avalanche photodiode, charge carriers generated by photon absorption are accelerated by a strong electric field due to an applied reverse voltage. This leads to impact ionization and consequent current gain in a portion of the device referred to as the “multiplication region”.
Avalanche photodiodes have been made in various semiconductor material systems. Known designs include, for example, a p-i-n structure in a vertically layered geometry, in which an upper germanium portion contains the optical absorption region and a lower silicon portion contains the multiplication region.
Recently, we developed a modified germanium/silicon p-i-n design in which the multiplication region is laterally offset from the germanium absorption region. Our modified design is described in U.S. patent application Ser. No. 13/915,369, which is cited above and the entirety of which is hereby incorporated herein by reference.
One factor that limits the performance of avalanche photodiodes is the dark current, i.e., the current that arises in the absence of light due to factors such as leakage at the diode surface and thermal carrier generation. Because of dark current, light levels that fall below a minimum threshold cannot be detected. This is important for avalanche diodes in general, but especially important for SPADs.
To achieve still greater sensitivity of optical detection using avalanche diodes, there remains a need for new approaches to the suppression of dark current.